Gallium Nitride-Based Compound Semiconductor Light Emitting Device

ABSTRACT

This gallium nitride-based compound semiconductor light emitting device includes an n-type semiconductor layer, a light emitting layer, and a p-type semiconductor layer that are composed of gallium nitride-based compound semiconductors and are deposited in that order on a substrate, and further includes a negative electrode and a positive electrode that are in contact with the n-type semiconductor layer and the p-type semiconductor layer, respectively, wherein the positive electrode has a translucent electrode composed of a three-layer structure including a contact metal layer that contacts at least the p-type semiconductor layer, a current diffusion layer provided on the contact metal layer and having conductivity greater than that of the contact metal layer, and a bonding pad layer provided on the current diffusion layer, and a mixed positive electrode-metal layer including a metal that forms the contact metal layer is present in a positive electrode side surface of the p-type semiconductor layer.

TECHNICAL FIELD

The present invention relates to a gallium nitride-based semiconductordevice, and more particularly, to a gallium nitride-based compoundsemiconductor device provided with a translucent positive electrodehaving superior characteristics and productivity.

The present application claims priority on Japanese Patent ApplicationNo. 2004-156543 filed on May 26, 2004 and on US Provisional PatentApplication No. 60/577218 filed on Jun. 7, 2004, the contents of whichare incorporated herein by reference.

BACKGROUND ART

GaN-based compound semiconductor materials have recently attractedattention as semiconductor materials for short wavelength light-emittingdevices. GaN-based compound semiconductors are formed on substratescomposed of sapphire single crystals, various other oxides, or III-Vcompounds by a method such as metalloorganic chemical vapor deposition(MOCVD) or molecular beam epitaxy (MBE).

One of the characteristics of GaN-based compound semiconductor materialsis having low current diffusion in the horizontal direction. Althoughthis is thought to be due to the presence of large numbers ofdislocations penetrating from the substrate to the surface in epitaxialcrystals, the details of this are not fully understood. Moreover, inp-type GaN-based compound semiconductors, the resistivity is higher thanthe resistivity of n-type GaN-based compound semiconductors, there ishardly any horizontal spreading of current in the p layer in the case inwhich metal is simply deposited on the surface thereof, thereby in thecase of adopting an LED structure having a pn junction, light is onlyemitted downward directly beneath the positive electrode.

Consequently, current diffusivity is enhanced by electron beamirradiation or high-temperature annealing to lower the resistivity ofthe p layer. However, the cost of a device for electron beam irradiationis extremely high, making it incompatible with production costs. Inaddition, it is difficult to uniformly process the wafer surface. In thecase of high-temperature annealing as well, although it is necessary toemploy a process at a temperature of 900° C. or higher in order to yieldremarkable effects, there is the risk of decomposition of the crystalstructure of the GaN and deterioration of reverse voltagecharacteristics caused by the elimination of nitrogen.

In addition, it has also been proposed to deposite Ni and Au at 10 nmeach on the p layer for use as the positive electrode, and carry outalloying treatment in an oxygen atmosphere to promote lower resistanceof the p layer as well as form a translucent and ohmic positiveelectrode (see, for example, Japanese Patent No. 2803742).

However, alloying treatment in an oxygen atmosphere causes the formationof an oxide layer on the surface of the exposed n-type GaN layer, andhas an ohmic effect on the negative electrode. Moreover, Au/Nielectrodes that have been subjected to alloying treatment in an oxygenatmosphere have mesh structures, resulting in increased susceptibilityto the occurrence of emission unevenness, reduced mechanical strengthand the need to provide a protective film, which in turn leads toincreased production costs. Moreover, since the Ni is heat treated in anoxygen atmosphere, when Ni oxides cover the surface and a pad electrodeis formed on the translucent electrode, it has weak adhesive strengththereby preventing the obtaining of bonding strength.

In addition, it has also been proposed to form a Pt layer on the p layerfor use as the positive electrode followed by heat treatment in anatmosphere including oxygen to simultaneously carry out resistancereduction and alloying treatment of the p layer (see, for example,Japanese Unexamined Patent Application, First Publication No.H11-186605).

However, since this method also involves heat treatment in an oxygenatmosphere, it has the same problems as those described above. Moreover,although the translucent electrode must be considerably thin (5 nm orless) to obtain a satisfactory translucent electrode with Pt alone, thisresults in an increase in the electrical resistance of the Pt layer.Thus, even if resistance of the Pt layer is reduced by heat treatment,current spreading is poor and emission of light is not uniform, leadingto an increase in forward voltage (VF) as well as a decrease in emissionintensity.

DISCLOSURE OF THE INVENTION

In order to solve the aforementioned problems, the object of the presentinvention is to provide a gallium nitride-based compound semiconductorlight emitting device provided with a positive electrode havingsatisfactory translucency, low contact resistance and superior currentdiffusivity without requiring electron beam irradiation,high-temperature annealing or alloying treatment in an oxygenatmosphere. In the present invention, translucency refers totranslucency with respect to light in the wavelength range of 300 to 600nm.

The present invention provides the following inventions.

-   (1) A gallium nitride-based compound semiconductor light emitting    device includes an n-type semiconductor layer, a light emitting    layer, and a p-type semiconductor layer that are composed of gallium    nitride-based compound semiconductors and are deposited in that    order on a substrate, and further includes a negative electrode and    a positive electrode that are in contact with the n-type    semiconductor layer and the p-type semiconductor layer,    respectively, wherein the positive electrode has a translucent    electrode composed of a three-layer structure including a contact    metal layer that contacts at least the p-type semiconductor layer, a    current diffusion layer provided on the contact metal layer and    having conductivity greater than that of the contact metal layer,    and a bonding pad layer provided on the current diffusion layer, and    a mixed positive electrode-metal layer including a metal that forms    the contact metal layer is present in a positive-electrode-side    surface of the p-type semiconductor layer.-   (2) The gallium nitride-based compound semiconductor light emitting    device according to (1), wherein a thickness of the mixed positive    electrode-metal layer is 0.1 to 10 nm.-   (3) The gallium nitride-based compound semiconductor light emitting    device according to (1) or (2), wherein a percentage of the metal    that forms the contact metal layer in the mixed positive    electrode-metal layer to a total metal in the mixed positive    electrode-metal layer is 0.01 to 30 atomic percent.-   (4) The gallium nitride-based compound semiconductor light emitting    device according to any of (1) to (3), wherein a mixed    semiconductor-metal layer that includes a group III metal is present    in a p-type-semiconductor-layer-side surface of the contact metal    layer.-   (5) The gallium nitride-based compound semiconductor light emitting    device according to (4), wherein a thickness of the mixed    semiconductor-metal layer is 0.1 to 2.5 nm.-   (6) The gallium nitride-based compound semiconductor light emitting    device according to (4) or (5), wherein a percentage of the group    III metal in the mixed semiconductor-metal layer to a total metal in    the mixed semiconductor-metal layer is 0.1 to 50 atomic percent.-   (7) The gallium nitride-based compound semiconductor light emitting    device according to any of (1) to (6), wherein the contact metal    layer is of a platinum group metal or Ag.-   (8) The gallium nitride-based compound semiconductor light emitting    device according to any of (1) to (7), wherein the contact metal    layer is of platinum.-   (9) The gallium nitride-based compound semiconductor light emitting    device according to any of (1) to (8), wherein a thickness of the    contact metal layer is 0.1 to 7.5 nm.-   (10) The gallium nitride-based compound semiconductor light emitting    device according to any of (1) to (9), wherein a thickness of the    contact metal layer is 5 nm or less.-   (11) The gallium nitride-based compound semiconductor light emitting    device according to any of (1) to (10), wherein a thickness of the    contact metal layer is 0.5 to 2.5 nm.-   (12) The gallium nitride-based compound semiconductor light emitting    device according to any of (1) to (11), wherein the current    diffusion layer is of a metal selected from the group consisting of    gold, silver, copper, and an alloy including at least one thereof.-   (13) The gallium nitride-based compound semiconductor light emitting    device according to any of (1) to (12), wherein the current    diffusion layer is of gold.-   (14) The gallium nitride-based compound semiconductor light emitting    device according to any of (1) to (13), wherein a thickness of the    current diffusion layer is 1 to 20 nm.-   (15) The gallium nitride-based compound semiconductor light emitting    device according to any of (1) to (14), wherein a thickness of the    current diffusion layer is 10 nm or less.-   (16) The gallium nitride-based compound semiconductor light emitting    device according to any of (1) to (15), wherein a thickness of the    current diffusion layer is 3 to 6 nm.-   (17) The gallium nitride-based compound semiconductor light emitting    device according to any of (1) to (16), wherein the bonding pad    layer includes a eutectic solder material.-   (18) The gallium nitride-based compound semiconductor light emitting    device according to any of (1) to (17), wherein the bonding pad    layer includes Au, Sn, or a tertiary solder alloy including Au and    Sn.

A translucent positive electrode of the present invention, which isprovided with a contact metal layer in the form of a thin layer of ametal such as a platinum group metal having low contact resistance witha p-type GaN-based compound semiconductor, and a current diffusion layerprovided on the contact metal layer and having a larger conductivitythan that of the contact metal layer, has improved spread of current inan in-plane direction of the electrode, and as a result, a highluminance light emitting device can be produced having a low forwardvoltage (VF) and uniform emission of light over the entire surface ofthe positive electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing showing a cross-sectional structure of agallium nitride-based compound semiconductor light emitting device ofthe present invention.

FIG. 2 is a schematic drawing showing a cross-sectional structure of agallium nitride-based compound semiconductor light emitting device of anembodiment of the present invention.

FIG. 3 is a schematic drawing showing an overhead view of a galliumnitride-based compound semiconductor light emitting device of anembodiment of the present invention.

FIG. 4 is a drawing showing an enlarged view of the vicinity of thejunction between a p-type semiconductor layer and a contact metal layerof a gallium nitride-based compound semiconductor light emitting deviceof the present invention.

FIG. 5 is an example of an EDS analysis chart of cross-sectional TEMperformed on a contact metal layer of a gallium nitride-based compoundsemiconductor light emitting device of the present invention.

FIG. 6 is an example of an EDS analysis chart of cross-sectional TEMperformed on a p-type semiconductor layer of a gallium nitride-basedcompound semiconductor light emitting device of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The following provides an explanation of preferred embodiments of thepresent invention with reference to the drawings. However, the presentinvention is not limited to each of the following embodiments, and forexample, the constituents of these embodiments may be suitably combined.

FIG. 1 is a schematic drawing showing the cross-section of a galliumnitride-based compound semiconductor light emitting device 100 having atranslucent positive electrode of the present invention.

This gallium nitride-based compound semiconductor light emitting device100 of the present invention has a gallium nitride-based compoundsemiconductor layer 2 formed on a substrate 1 interposed with a bufferlayer 6 between them, and a translucent positive electrode 10 of thepresent invention formed thereon.

Gallium nitride-based compound semiconductor layer 2 is composed with ahetero junction structure including, for example, an n-typesemiconductor layer 3, a light emitting layer 4 and a p-typesemiconductor layer 5.

A negative electrode 20 is formed in a portion of the n-typesemiconductor layer 3, and the translucent positive electrode 10 isformed in a portion of the p-type semiconductor layer 5.

In addition, the translucent positive electrode 10 has three layersconsisting of a contact metal layer 11, a current diffusion layer 12,and a bonding pad layer 13.

Small contact resistance with the p-type semiconductor layer 5 isessential for the required performance of the contact metal layer 11.Moreover, superior optical transmissivity is required for face-upmounting type light emitting devices in which light from the lightemitting layer 4 is extracted from the electrode side.

A material of the contact metal layer 11 is preferably a platinum groupmetal such as platinum (Pt), ruthenium (Ru), osmium (Os), rhodium (Rd),iridium (Ir), or palladium (Pd), or silver (Ag) from the viewpoint ofcontact resistance with the p layer. Among these, Pt is particularlypreferable since Pt has a high work function and allows to obtain asatisfactory ohmic contact without heating with the p-type GaN-basedcompound semiconductor layer which is not subjected to high-temperatureheat treatment and has comparatively high resistance.

In the case in which the contact metal layer 11 includes a platinumgroup metal, it is necessary to make its thickness extremely thin fromthe viewpoint of optical transmissivity. The thickness of the contactmetal layer 11 is preferably within a range from 0.1 to 7.5 nm. In thecase in which the thickness is less than 0.1 nm, it is difficult toobtain a stable thin layer. In the case in which the thickness exceeds7.5 nm, translucency decreases, and a thickness of 5 nm or less is morepreferable. In addition, the thickness of the contact metal layer 11 isparticularly preferably 0.5 to 2.5 nm in consideration of depositionstability and a decrease in the translucency caused by a subsequentdeposition of the current diffusion layer 12.

However, since reducing the thickness of the contact metal layer 11 inthis manner causes an increase in the electrical resistance in theplanar direction of the contact metal layer 11, and the p layer has acomparatively high resistance, the current flows only in a periphery ofthe bonding pad layer 13 serving as a current injection area. As aresult, light emission pattern is not uniform and emission outputdecreases.

Therefore, by arranging the current diffusion layer 12 which is composedof a metal thin film having high optical transmissivity and highconductivity on the contact metal layer 11 as a means of compensatingfor the current diffusivity of the contact metal layer 11, the currentis able to spread uniformly without significantly impairing a lowcontact resistance and a optical transmissivity of the platinum groupmetal. As a result, a light emitting device having a high emissionoutput can be obtained.

A material of the current diffusion layer 12 is preferably a metalhaving high conductivity, such as a metal selected from the groupconsisting of gold, silver, copper, and an alloy that includes at leastone type of these metals. Gold is particularly preferable due to itshigh optical transmissivity when formed into a thin film.

A thickness of the current diffusion layer 12 is preferably 1 to 20 nm.In the case in which the thickness is less than 1 nm, current diffusioneffects are not adequately demonstrated. In the case in which thethickness exceeds 20 nm, an optical transmissivity of the currentdiffusion layer 12 decreases remarkably, resulting in the risk of adecrease in emission output. A thickness of 10 nm or less is morepreferable.

Moreover, when the thickness is made to be within a range from 3 to 6nm, a balance between the effects of an optical transmissivity and acurrent diffusion of the current diffusion layer 12 becomes optimal,thereby, together with the aforementioned contact metal layer,high-output emission that is uniform over the entire surface of thepositive electrode can be obtained.

In a gallium nitride-based compound semiconductor light emitting deviceof the present invention, a mixed positive electrode-metal layerincluding a metal that forms the aforementioned contact metal layer ismade to be present in a positive-electrode-side surface of the p-typesemiconductor layer 5. As a result of employing this constitution, aneffect of lowering a contact resistance between the positive electrode10 and the p-type semiconductor layer 5 is obtained.

In the present invention, the “mixed positive electrode-metal layer” isdefined as a layer including a metal that forms the contact metal layerin the p-type semiconductor layer 5.

A thickness of the mixed positive electrode-metal layer is preferably0.1 to 10 nm. In the case in which the thickness is less than 0.1 orgreater than 10 nm, it is difficult to obtain a low contact resistance.The thickness is more preferably 1 to 8 nm to obtain even better contactresistance.

In addition, a ratio of the metal that forms the contact metal layerincluded in the mixed positive electrode-metal layer is preferably 0.01to 30 atomic percent relative to the total amount of metal. In the casein which the ratio is less than 0.01 atomic percent, it is difficult toobtain a low contact resistance, and in the case in which the ratioexceeds 30 atomic percent, there is a risk of causing a poorsemiconductor crystallinity. The ratio is preferably 1 to 20 atomicpercent. Here, the mixed positive electrode-metal layer may also includea metal that forms a reflecting layer. In this case, the aforementionedratio is evaluated as a value of the amount of the metal that forms thecontact metal layer and the amount of the metal that forms thereflecting layer.

The thickness of the mixed positive electrode-metal layer and the ratioof the metal that forms the positive electrode included therein can bemeasured by an EDS analysis of a cross-sectional TEM that is known amongpersons with ordinary skills in the art. Namely, the EDS analysis of thecross-sectional TEM is performed at several points, for example 5points, in a direction of thickness from an upper surface (surface onthe positive electrode side) of the p-type semiconductor layer, and themetals included along with their amounts are determined from the chartat each point. When the measurement at five points is not adequate fordetermining the thickness, measurements should be made at additionalpoints to determine the thickness.

Moreover, in the case in which a mixed semiconductor-metal layerincluding a metal that composes a semiconductor is made to be present ina semiconductor-side surface of the contact metal layer 11 of thepositive electrode, a contact resistance can be lowered even more,thereby making this preferable. Namely, in the present invention, the“mixed semiconductor-metal layer” is defined as a layer that includes ametal that composes a semiconductor in the contact metal layer.

A thickness of the mixed semiconductor-metal layer is preferably 0.1 to3 nm. In the case in which the thickness is less than 0.1 nm, the effectof lowering the contact resistance is not remarkable. In the case inwhich the thickness exceeds 3 nm, the optical transmissivity decreases,which is not preferable. The thickness is more preferably 1 to 2.5 nm.

In addition, a ratio of the metal that composes the semiconductorincluded in the mixed semiconductor-metal layer is preferably 0.1 to 50atomic percent relative to the total amount of metal. In the case inwhich the ratio is less than 0.1 atomic percent, the effect of loweringthe contact resistance is not remarkable. In the case in which the ratioexceeds 50 atomic percent, there is the risk of decreasing the opticaltransmissivity. This ratio is more preferably 1 to 20 atomic percent.

Measurement of the thickness of the mixed semiconductor-metal layer andthe content of the metal that composes the semiconductor can be carriedout by an EDS analysis of a cross-sectional TEM in the same manner asfor the mixed positive electrode-metal layer.

There are no particular limitations on the method used to deposit thecontact metal layer 11 and the current diffusion layer 12, and a knownmethod such as vacuum deposition or sputtering can be used.

Various structures using various materials are known for the bonding padlayer 13 that composes the bonding pad area, and these known structuresand materials can be used without any particular limitations. However,it is desirable to use a material having satisfactory adhesion with thecurrent diffusion layer, and it is necessary that a thickness besufficiently thick so as not to impart damage to the contact metal layer11 or the current diffusion layer 12 by a stress generated duringbonding. In addition, an uppermost layer preferably includes a materialhaving satisfactory adhesion with a bonding ball.

As shown in FIG. 1, the translucent positive electrode 10 of the presentinvention can be used without any limitations for a galliumnitride-based compound semiconductor light emitting device known in theprior art, in which the gallium nitride-based compound semiconductorlayer 2 is deposited on the substrate 1 with the buffer layer 6interposed therebetween, and the n-type semiconductor layer 3, the lightemitting layer 4, and the p-type semiconductor layer 5 are formed.

A known substrate material can be used without any limitations for thesubstrate 1, examples of which include sapphire single crystal (Al₂O₃; Asurface, C surface, M surface, and R surface), spinel single crystal(MgAl₂O₄), ZnO single crystal, LiAlO₂ single crystal, LiGaO₂ singlecrystal, MgO single crystal, and other oxide single crystals, Si singlecrystal, SiC single crystal, GaAs single crystal, AlN single crystal,GaN single crystal, and ZrB₂ single crystal and other boride singlecrystals. Here, there are no particular limitations on a plane directionof the substrate 1. In addition, substrates having a surface exactlyoriented to a crystal plane, or an off-angle may also be used.

Various types of structures of the n-type semiconductor layer 3, thelight emitting layer 4, and the p-type semiconductor layer 5 are known,and these known structures can be used without limitation. Inparticular, although a typical concentration is used for the carrierconcentration of the p-type semiconductor layer 5, the translucentpositive electrode of the present invention can also be applied to as anelectrode for the p-type semiconductor layers having comparatively lowcarrier concentrations, for example, a carrier concentration of about1×10¹⁷ cm⁻³.

Semiconductors of various compositions represented by the generalformula Al_(x)In_(y)Ga_(1−x−y)N (0≦x<1, 0≦y<1, 0≦x+y<1) are known forthe gallium nitride-based compound semiconductors that form thelight-emitting device, and various compositions of semiconductorsrepresented by the general formula Al_(x)In_(y)Ga_(1−x−y)N (0≦x<1,0≦y<1, 0≦x+y<1) can be used without limitation for the n-typesemiconductor layer, the light emitting layer, and the p-typesemiconductor layer in the present invention.

There are no particular limitations on a growth method of these galliumnitride-based compound semiconductors, and all known methods for growinggroup III nitride semiconductors can be applied, examples of whichinclude metalloorganic chemical vapor deposition (MOCVD), hydride vaporphase epitaxy (HVPE), and molecular beam epitaxy (MBE). A preferablegrowth method is MOCVD from the viewpoint of a film thicknesscontrollability and a mass production.

In the MOCVD method, hydrogen (H₂) or nitrogen (N₂) is used as a carriergas, trimethyl gallium (TMG) or triethyl gallium (TEG) is used as a Gasource serving as a group III material, trimethyl aluminum (TMA) ortriethyl aluminum (TEA) is used as an Al source, trimethyl indium (TMI)or triethyl indium (TEI) is used as an In source, and ammonia (NH₃) orhydrazine (N₂H₄) is used as a N source serving as a group V material. Inaddition, examples of dopants used as an n type include Si material ofmonosilane (SiH₄) or disilane (Si₂H₆) and a Ge material of germane(GeH₄), while examples of dopants used for the p type include a Mgmaterial of bis-cyclopentadienyl magnesium (Cp₂Mg) orbis-ethylcyclopentadienyl magnesium ((EtCp)₂Mg).

In order to form the negative electrode 20 in contact with the n-typesemiconductor layer 3 of the gallium nitride-based compoundsemiconductor 2 in which the n-type semiconductor layer 3, the lightemitting layer 4, and the p-type semiconductor layer 5 are sequentiallydeposited on the substrate 1, portions of the light emitting layer 4 andthe p-type semiconductor layer 5 are removed to expose the n-typesemiconductor layer 3. Subsequently, the translucent positive electrode10 of the present invention is then formed on the remaining p-typesemiconductor layer 5, and the negative electrode 20 is formed on theexposed n-type semiconductor layer 3. Negative electrodes having variouscompositions and structures are known for the negative electrode 20, andthese known negative electrodes can be used without limitation.

EXAMPLE

Next, the following provides a more detailed explanation of the presentinvention through its embodiments, however the present invention is notlimited to these embodiments.

Example

FIG. 2 is a schematic drawing showing the cross-section of a galliumnitride-based compound semiconductor light emitting device 200 producedin the present example. FIG. 3 is a schematic drawing of its overheadview. A base layer 3 a composed of undoped GaN and having a thickness of3 μm; a Si-doped n-type GaN contact layer 3 b having a thickness of 2μm; an n-type In_(0.1)Ga_(0.9)N cladding layer 3 c having a thickness of0.03 μm; a light emitting layer 4 with a multiple-quantum-well structureformed by depositing an Si-doped GaN barrier layer having a thickness of0.03 μm and an In_(0.2)Ga_(0.8)N well layer having a thickness of 2.5 nmfive times and finally providing a barrier layer; an Mg-doped p-typeAl_(0.07)Ga_(0.93)N cladding layer 5 a having a thickness of 0.05 μm;and an Mg-doped p-type GaN contact layer 5 b having a thickness of 0.15μm were deposited sequentially on a sapphire substrate 1 with a bufferlayer 6 composed of AlN intervening.

A positive electrode 10 composed of a contact metal layer 11 composed ofPt and having a thickness of 1.5 nm, a current diffusion layer 12composed of Au and having a thickness of 5 nm, and a bonding pad layer13 having a five-layer structure consisting of Au, Ti, Al, Ti, and Au inthat order was formed on the p-type GaN contact layer 5 b of the galliumnitride-based compound semiconductor 2. Thicknesses of these layers were50, 20, 10, 100, and 200 nm, respectively.

Next, a negative electrode 20 having a bilayer structure consisting ofTi and Au was formed on the n-type GaN contact layer 3 b to obtain alight emitting device in which a light emission face is on asemiconductor side. Overhead views of the positive electrode 10 and thenegative electrode 20 were as shown in FIG. 3.

In a light emitting device employing this structure, a carrierconcentration in the n-type GaN contact layer 3 b was 1×10¹⁹ cm⁻³, anamount of doped Si in the GaN barrier layer was 1×10¹⁸ cm⁻³, a carrierconcentration in the p-type GaN contact layer 5 b was 5×10¹⁸ cm⁻³, andan amount of doped Mg in the p-type AlGaN cladding layer 5 a was 5×10¹⁹cm⁻³.

Deposition of the gallium nitride-based compound semiconductor layer 3was carried out by MOCVD method under ordinary conditions well known inthe relevant technical field. In addition, the positive electrode 10 andthe negative electrode 20 were formed according to the followingprocedure.

A portion of the n-type GaN contact layer 3 b that forms the negativeelectrode was initially exposed by a reactive ion etching according tothe following procedure.

First, an etching mask was formed on the p-type semiconductor layer 5.The forming procedure was as described below. After uniformly coating aresist over an entire surface, the resist was removed over a regionlarger than a periphery of the positive electrode region using a knownlithography technology. The substrate was then placed in a vacuumdeposition device, and Ni and Ti were deposited at film thicknesses ofabout 50 nm and 300 nm, respectively, by an electron beam method at apressure of 4×10⁻⁴ Pa. The metal film other than the positive electroderegion was then removed along with the resist by a liftoff technology.

Next, the semiconductor deposited substrate was then placed on anelectrode in an etching chamber of a reaction ion sputtering device, andafter reducing a pressure in the etching chamber to 10⁻⁴ Pa, Cl₂ gas asan etching gas was fed into the chamber and etching was carried outuntil the n-type GaN contact layer 3 b was exposed. Following etching,the deposited substrate was taken out of the reaction ion etching deviceand the etching mask was removed with nitric acid and hydrofluoric acid.

Next, a contact metal layer 11 including Pt and a current diffusionlayer 12 including Au were formed only at a region in which the positiveelectrode was formed on the p-type GaN contact layer 5 b using a knownphotolithography and liftoff technologies. In a formation of the contactmetal layer 11 and the current diffusion layer 12, the substrate 1 onwhich gallium nitride-based compound semiconductor layer 3 was depositedwas first placed in a vacuum deposition device, and at first Pt at 1.5nm and secondary Au at 5 nm were deposited on the p-type GaN contactlayer 5 b. After taking out of the vacuum chamber, the depositedsubstrate was treated in accordance with a known procedure ordinarilyreferred to as a liftoff, after which a first layer including Au, asecond layer including Ti, a third layer including Al, a fourth layerincluding Ti, and a fifth layer including Au were sequentially depositedon a portion of the current diffusion layer 12 to form a bonding padlayer 13 using the same procedure. A positive electrode 10 of thepresent invention was thus formed on the p-type GaN contact layer 5 b inthis manner.

The positive electrode formed by this method demonstrated translucency,and had an optical transmissivity of 60% in a 470 nm wavelength range.Here, the optical transmissivity was measured by using a sample in whichthe same contact metal layer and the current diffusion layer were formedto sizes for measurement of the optical transmissivity.

FIG. 4 shows an enlarge view of a vicinity of a junction between thep-type semiconductor layer and the contact metal layer of the galliumnitride-based compound semiconductor light emitting device of thepresent invention.

As shown in FIG. 4, in a gallium nitride-based compound semiconductorlight emitting device of the present invention, a mixed positiveelectrode-metal layer 15 b including a metal that forms the contactmetal layer 11 is present in a vicinity of a contact-metal-layer-11-sideinterface of the p-type semiconductor layer 5 b, while a mixedsemiconductor-metal layer 15 a including a metal that composessemiconductor layer 2 is present in a vicinity of ap-type-semiconductor-layer-5 b-side interface on of the contact metallayer 11. Namely, the mixed semiconductor-metal layer 15 a and the mixedpositive electrode-metal layer 15 b form a mutual diffusion layer 15 atthe interface of the junction between the contact metal layer 11 and thep-type semiconductor layer 5 b. The presence of this mutual diffusionlayer 15 makes it possible to demonstrate the effect of obtaining ajunction interface having superior current diffusivity with lowresistance.

Furthermore, as a result of an EDS analysis of a cross-sectional TEM, athickness of the mixed semiconductor-metal layer 15 a was found to be1.5 nm, and a ratio of Ga in the layer was estimated to be 1 to 20atomic percent with respect to a total amount of metal (Pt+Au+Ga). Inaddition, a thickness of the mixed positive electrode-metal layer 15 bwas 6.0 nm, a positive electrode material present was Pt that composedthe contact metal layer 11, and its ratio in the layer was estimated tobe 1 to 10 atomic percent relative to a total amount of metal (Pt+Ga).Here, FIG. 5 shows an example of an EDS analysis chart of across-sectional TEM of the contact metal layer, while FIG. 6 shows anexample of an EDS analysis chart of a cross-sectional TEM of the contactlayer 5 b.

As shown in FIG. 5, Pt that forms the contact metal layer 11 is presentin the mixed positive electrode-metal layer 15 b located in the vicinityof the contact-metal-layer-11-side interface of the p-type semiconductorlayer 5 b composed of p-type GaN, while on the other hand, Ga thatcomposes the p-type semiconductor layer 5 b composed of GaN is presentin the mixed semiconductor-metal layer 15 a located in the vicinity ofthe p-type-semiconductor-layer-5 b-side interface of contact metal layer11.

Here, Cu peaks in the chart are the result of the X-rays used formeasurement.

Next, a negative electrode 20 was formed on the exposed n-type GaNcontact layer 3 b according to the following procedure. After uniformlycoating a resist over an entire surface, the resist was removed from aregion where the negative electrode was formed on the exposed n-type GaNcontact layer using a known lithography technology to form the negativeelectrode composed of Ti at 100 nm and Au at 200 nm in order from thesemiconductor side using a routinely used vacuum deposition. The resistwas subsequently removed by a known method.

A thickness of the substrate on which the positive electrode 10 and thenegative electrode 20 were formed in this manner was reduced to 80 μm bygrinding and polishing a rear surface of the substrate, and afterforming ruled lines on the semiconductor layer side using a laserscrubber, the substrate was split apart and cut into square chipsmeasuring 350 μm on a side. Continuing, when a forward voltage of thesechips was measured using a needle probe while supplying power andapplying current of 20 mA, the forward voltage was found to be 2.9 V.

Subsequently, when the chips were mounted in a TO-18 lead package andmeasured for an emission output with a tester, the emission output at anapplied current of 20 mA was 4 mW. In addition, the emissiondistribution of the emitting surface was confirmed to be such that lightwas emitted over the entire surface above the positive electrode.

Comparative Example 1

A gallium nitride-based compound semiconductor light emitting device wasproduced in the same manner as Example with the exception of notproviding the current diffusion layer. A forward voltage and an emissionoutput of this light emitting device were 3.1 V and 3.7 mW,respectively. Observation of its light emitting surface revealed thatemission above the electrode was limited to only a periphery of thebonding pad layer and a portion that centered about a line passingthrough the negative electrode from the bonding pad layer.

The reason of this was most likely high electrical resistance in theplanar direction of the contact metal layer that prevented the spread ofcurrent on the contact metal layer.

Comparative Example 2

A gallium nitride-based compound semiconductor light emitting device wasproduced in the same manner as Example with the exception of notproviding the current diffusion layer and making a thickness of thecontact metal layer 12 nm. A forward voltage and an emission output ofthis light emitting device were 2.9 V and 3.0 mW, respectively.Observation of its light emitting surface revealed that, althoughemission of light was confirmed over the entire surface as in Example,an optical transmissivity of the contact metal layer had decreased toabout 30%, resulting in a decrease in emission output.

INDUSTRIAL APPLICABILITY

An electrode for a gallium nitride-based compound semiconductor lightemitting device provided by the present invention is useful as apositive electrode of a translucent gallium nitride-based compoundsemiconductor light emitting device.

1. A gallium nitride-based compound semiconductor light emitting devicecomprising an n-type semiconductor layer, a light emitting layer, and ap-type semiconductor layer that are composed of gallium nitride-basedcompound semiconductors and are deposited in that order on a substrate,and further comprising a negative electrode and a positive electrodethat are in contact with the n-type semiconductor layer and the p-typesemiconductor layer, respectively, wherein the positive electrode has atranslucent electrode composed of a three-layer structure including acontact metal layer that contacts at least the p-type semiconductorlayer, a current diffusion layer provided on the contact metal layer andhaving conductivity greater than that of the contact metal layer, and abonding pad layer provided on the current diffusion layer, and a mixedpositive electrode-metal layer including a metal that forms the contactmetal layer is present in a positive-electrode-side surface of thep-type semiconductor layer.
 2. The gallium nitride-based compoundsemiconductor light emitting device according to claim 1, wherein athickness of the mixed positive electrode-metal layer is 0.1 to 10 nm.3. The gallium nitride-based compound semiconductor light emittingdevice according to claim 1, wherein a percentage of the metal thatforms the contact metal layer in the mixed positive electrode-metallayer to a total metal in the mixed positive electrode-metal layer is0.01 to 30 atomic percent.
 4. The gallium nitride-based compoundsemiconductor light emitting device according to claim 1, wherein amixed semiconductor-metal layer that includes a group III metal ispresent in a p-type-semiconductor-layer-side surface of the contactmetal layer.
 5. The gallium nitride-based compound semiconductor lightemitting device according to claim 4, wherein a thickness of the mixedsemiconductor-metal layer is 0.1 to 2.5 nm.
 6. The gallium nitride-basedcompound semiconductor light emitting device according to claim 4,wherein a percentage of the group III metal in the mixedsemiconductor-metal layer to a total metal in the mixedsemiconductor-metal layer is 0.1 to 50 atomic percent.
 7. The galliumnitride-based compound semiconductor light emitting device according toclaim 1, wherein the contact metal layer is of a platinum group metal orAg.
 8. The gallium nitride-based compound semiconductor light emittingdevice according to claim 1, wherein the contact metal layer is ofplatinum.
 9. The gallium nitride-based compound semiconductor lightemitting device according to claim 1, wherein a thickness of the contactmetal layer is 0.1 to 7.5 nm.
 10. The gallium nitride-based compoundsemiconductor light emitting device according to claim 1, wherein athickness of the contact metal layer is 5 nm or less.
 11. The galliumnitride-based compound semiconductor light emitting device according toclaim 1, wherein a thickness of the contact metal layer is 0.5 to 2.5nm.
 12. The gallium nitride-based compound semiconductor light emittingdevice according to claim 1, wherein the current diffusion layer is of ametal selected from the group consisting of gold, silver, copper, and analloy including at least one thereof.
 13. The gallium nitride-basedcompound semiconductor light emitting device according to claim 1,wherein the current diffusion layer is of gold.
 14. The galliumnitride-based compound semiconductor light emitting device according toclaim 1, wherein a thickness of the current diffusion layer is 1 to 20nm.
 15. The gallium nitride-based compound semiconductor light emittingdevice according to claim 1, wherein a thickness of the currentdiffusion layer is 10 nm or less.
 16. The gallium nitride-based compoundsemiconductor light emitting device according to claim 1, wherein athickness of the current diffusion layer is 3 to 6 nm.
 17. The galliumnitride-based compound semiconductor light emitting device according toclaim 1, wherein the bonding pad layer includes a eutectic soldermaterial.
 18. The gallium nitride-based compound semiconductor lightemitting device according to claim 1, wherein the bonding pad layerincludes Au, Sn, or a tertiary solder alloy including Au and Sn.